Foot Furnace premium bed slippers have been designed using the same principles as a sleeping bag. Foot Furnace Bed Slippers (ONLY TO BE USED IN BED) are made to last and constructed using the highest quality materials including fully breathable permeable nylon to keep your feet at their optimal temperature without any perspiration/transpiration or natural loss of heat. No batteries, electricity, or manual heating is required. One size FITS ALL, up to approximately size 11. Velcro Adjustment allows you to easily still wear socks underneath should you suffer from extremely cold feet Or Have Small Feet. The slippers also feature two useful Velcro straps allowing you to adjust the fit comfortably to your own feet. The soft fabric slipper design combined with premium filling make these top-rated bed slippers the preferred choice by customers over hot water bottles and wheat bags.